Comminution (size reduction by mechanical means) of crystalline solids using wet milling techniques such as ball milling or media milling processes is a common technique used in the paint and pigment industry and has recently been exploited for producing small (&lt;1 .mu.m) size particle dispersions of photographic materials, for example, see U.S. Pat. No. 4,940,654 to Diehl et al. In such milling applications, milling media are generally selected from a variety of dense materials, such as steel, ceramic or glass. In ball milling processes, both milling efficiency and attrition-related contamination are generally thought to be proportional to media density. Higher viscosity dispersions often require very dense media, such as stainless steel. Media geometries may vary depending on the application, although spherical or cylindrical beads are most commonly used.
Dispersions prepared by these techniques are typically stabilized using a surface agent to prevent agglomeration. In general, it is desirable to obtain the smallest possible particle size while minimizing attrition-related contamination from milling equipment and milling media. Such goals are often contradictory, i.e., the increased energy required to achieve a small particle size often results in excessive levels of metallic, ceramic or other types of contamination. High intensity milling is also desirable to maximize milling efficiencies, (i.e., rate of size reduction).
Attrition-related contamination in compounds useful in imaging elements dispersions (filter dyes, sensitizing dyes, couplers, antifoggants, etc.) can result in both physical and sensitometric defects. Contamination resulting from the milling process is usually present in the form of dissolved species or particulates of comparable sizes to dispersed product particles. Given this, separation of the contaminant particles from the product particles by filtration is generally ineffective. It is considered preferable to adjust formulation and process parameters and materials to minimize the generation of contaminants.
Attrition from the milling process can also result in chemical alteration of the product dispersion. Many types of ceramic and glass milling media contain metal oxides which release hydroxide ions into the dispersion and increase product pH. Such pH changes are undesirable since this can affect dispersion stability and change milling performance.
A further disadvantage of attrition is the excessive wear of milling media and mill components which can degrade milling performance and increase manufacturing maintenance costs. Most types of conventional media also require preconditioning to achieve a steady rate of wear.